The invention relates to screw compressors, and more particularly to bearing arrangements for screw compressors.
The rotors of a screw compressor are supported by bearings at both the suction end and the discharge end of the compressor. In a typical bearing arrangement, there is one radial bearing supporting each end of the rotor and at least one axial bearing supporting the discharge end of the rotor. This arrangement is used for both rotors. FIG. 1 illustrates a typical prior art bearing arrangement for a screw compressor 10.
As shown in FIG. 1, the compressor 10 includes two rotors 14 and 18 mounted for rotation between a suction end 22 and a discharge end 26. At the suction end 22, the first and second rotors 14, 18 are housed in a suction housing 30 and are supported by radial bearings 34 and 38. The suction housing 30 is connected to one end of a rotor housing 42 that surrounds portions of the rotors 14, 18. A discharge housing 46 is connected to the other end of the rotor housing 42. Radial bearings 50 and 54 support the discharge ends of the rotors 14, 18 inside the discharge housing 46.
Typically, each of the radial bearings 34, 38, 50, and 54 are rolling element bearings. When mounted, the separable inner race of each bearing 34, 38, 50, and 54 is pressed onto the ends of the respective rotor shafts, while the outer race with the rollers is retained in the respective suction housing 30 or discharge housing 46. Due to the tolerances, the fit between the outer race of the bearings 34 and 38 and the suction housing 30 is often a transition fit (slip fit and press fit), so it is common to axially fix each outer race between a shoulder 58 of the suction housing 30 on one end, and a snap ring 59 on the other end.
Likewise, the fit between the outer race of the bearings 50 and 54 and the discharge housing 46 is often a slip fit, so each outer race is axially fixed between a shoulder 60 on one end, and a snap ring 61 on the other end. The snap rings 59 and 61 are retained in grooves that are cast or machined in the respective suction housing 30 and discharge housing 46.
To accommodate the axial movement of the rotors 14, 18, axial bearings 66, 70, and 74 are used at the discharge end 26. Two of the axial bearings 66 and 70 are mounted in tandem relation on the first rotor 14, while the second rotor 18 has only one of the axial bearing 74 mounted thereon. Just as with the radial bearings 50, 54, the inner races of the axial bearings 66, 70, and 74 are pushed onto the respective rotor shafts, and the outer races are free to rotate. The axial bearings 66 and 70 are axially fixed between an end surface 78 of the discharge housing 46 on one end, and a thrust collar 82 on the other end. The thrust collar 82 is shrunk onto the rotor shaft, as is understood by those skilled in the art. Likewise, the axial bearing 74 is axially fixed between the end surface 78 on one end, and a thrust collar 86 on the other end.
The compressor 10 can also include a pair of reverse-thrust or backup bearings 90. The backup bearings 90 each have a spring-loaded outer race, in the form of a spring 91 that is retained between the bearing 90 and a cup 92, that biases the backup bearings 90 into engagement with a step in the respective rotors 14, 18. This spring bias is intended to keep the outer races of the axial bearings 66 and 74 securely seated against the end surface 78 during startup of the compressor 10, thereby substantially preventing any relative rotation or movement between the outer races of the axial bearings 66 and 74 and the end surface 78. While also serving other purposes, this backup bearing arrangement eliminates the need to mechanically anti-rotate the outer races of the axial bearings 66 and 74 with pins, keys, or other known anti-rotation devices. An axial bearing cover 94 is mounted to the end surface 78 of the discharge housing 46 to cover and protect the axial bearings 66, 70, 74, and 90 and to provide a fixed engagement surface for the spring-loaded backup bearings 90.
The prior-art bearing arrangement described above has some drawbacks. For example, when the compressor 10 is started, the spring bias of the backup bearings 90 is often not enough to keep the outer races of the bearings 66 and 74 seated securely against the end surface 78 of the discharge housing 46. This allows the outer races of the bearings 66 and 74 to rotate or vibrate relative to the end surface 78 of the discharge housing 46. Because the end surface 78 is typically a softer material (e.g., cast iron) than the material used for the outer race of the bearings 66 and 74 (e.g., steel), the rotation and vibration of the bearings 66 and 74 results in wearing and grooving in the end surface 78. The wearing and grooving can be further accentuated by non-perfect parallel seating of the bearings 66 and 74 against the end surface 78. Testing has shown wear rates on the order of one micrometer per one-thousand hours of compressor operation.
Large amounts of wearing and grooving result in increased axial rotor endplay with respect to the end surface 78 of the discharge housing 46. The increased axial end-play results in a loss of compressor performance and increased discharge temperatures, both of which decrease the overall efficiency of the compressor 10.
Using backup bearings 90 also creates other disadvantages. For example, the compressor 10 must be larger to accommodate the backup bearings 90, and the backup bearings 90 add to the overall cost of the compressor 10. These disadvantages, while tolerable if the backup bearings 90 perform as intended, are exacerbated when the backup bearings 90 fail to prevent the wearing and grooving that causes increased axial rotor end-play. Of course, the size of the backup bearings 90 can be increased to include a larger spring force to eliminate wearing and grooving, however, such larger backup bearings would reduce the life of the axial bearings 66, 70, and 74 due to the larger thrust force.
The present invention overcomes these and other problems by providing a wear-preventing and positioning device for combined axial and radial bearing arrangements in the discharge end of a compressor. The device substantially eliminates wearing and grooving on the discharge housing without the use of spring-loaded backup bearings or other anti-rotation devices coupled to the outer races of the axial bearings. Therefore, the overall size and cost of the compressor is greatly reduced. The single device also axially positions the radial bearings and the axial bearings on the discharge side. Therefore, the snap rings are also eliminated, further reducing the size and cost of the compressor.
The device is a thin, hard, specially-configured and flattened strip of material that fits over both rotor shafts and abuts the end surface of the discharge housing. One face of the strip axially fixes the radial bearings while the opposing face axially fixes the axial bearings. The strip is sandwiched between the outer race of the axial bearings and the end surface of the discharge housing to eliminate direct contact between the axial bearings and the discharge housing. Because the strip is approximately the same hardness as the material used for the outer races of the axial bearings, rotation or vibration of the axial bearings will not create significant wearing or grooving in the strip. The need for spring-loaded backup bearings to prevent rotation of the axial bearings is therefore eliminated.
Additionally, the special configuration of the strip substantially fixes the strip with respect to the discharge housing so that any movement or vibration of the axial bearings will not cause relative movement between the strip and the discharge housing. This substantially eliminates wear on the discharge housing, thereby eliminating the potential for increased axial rotor end-play.
More specifically, the invention provides a screw compressor including a suction end, a discharge end, first and second rotors mounted for rotation between the suction end and the discharge end, and a discharge housing at the discharge end. The discharge housing surrounds a portion of the first and second rotors and includes an end surface. The compressor further includes a first axial bearing supporting the first rotor, and a wear-preventing member sandwiched between the end surface of the discharge housing and the first axial bearing such that no portion of the first axial bearing contacts the end surface.
In another aspect of the invention, the compressor further includes a second axial bearing supporting the second rotor. The wear-preventing member is also sandwiched between the end surface of the discharge housing and the second axial bearing such that no portion of the second axial bearing contacts the end surface.
In yet another aspect of the invention, the screw compressor further includes a first radial bearing in the discharge housing for supporting the first rotor. The first radial bearing has a suction-side face and a discharge-side face, the suction-side face being seated in the discharge housing. The wear-preventing member includes a first face abutting the end surface of the discharge housing and abutting at least a portion of the discharge-side face of the first radial bearing to retain the first radial bearing in the discharge housing.
In another aspect of the invention, the screw compressor also includes a second radial bearing in the discharge housing for supporting the second rotor. The second radial bearing has a suction-side face and a discharge-side face, the suction-side face being seated in the discharge housing. The first face of the wear-preventing member abuts at least a portion of the discharge-side face of the second radial bearing to retain the second radial bearing in the discharge housing. Preferably, the wear-preventing member is substantially figure-eight-shaped.
The invention also provides a method of assembling a screw compressor having first and second rotors and a discharge housing surrounding a portion of the first and second rotors. The discharge housing includes an end surface. The method includes inserting first and second radial bearings into the discharge housing to support the respective first and second rotors, placing a wear-preventing and positioning device against the end surface of the discharge housing to retain the radial bearings in the discharge housing, and mounting first and second axial bearings on the respective first and second rotors such that the first and second axial bearings abut the wear-preventing and positioning device. The wear-preventing and positioning device substantially prevents engagement between the first and second axial bearings and the end surface of the discharge housing.